Series of fossils in various taxa suggest that phyletic lines begin as primitive types during evolution and end up at higher level through progressive evolution. Although lower forms may have been perfectly adapted to their specific habitats, higher forms show increased complexity of organs and adaptiveness achieved through consistent natural selection. Present day mammals possess much more complex body organization than their Cretaceous ancestors.
So, the single phylogenetic lineage in which a primitive ancestral species gradually acquires complexity and specialization in response to the environmental stimuli is called Anagenesis, whereas the branching of the lines of descent is known as Cladogenesis.
Natural Selection produces adaptations in relation to the existing conditions but such adaptations may prove disadvantageous in future and hence increase in the complexity and perfection of adaptations is a continuous biological process that leads a species to higher level. Anagenesis has the following characteristic features:
S.W. Williston (1852-1918) proposed that in organisms having serially repeated body parts there is a tendency of reduction in the number and increase in specialisation of these parts during Anagenesis. In arthropods, crustaceans possess almost 19 pairs of serially arranged appendages which are modified for various purposes, namely, food handling, walking and swimming. But in higher arthropods such as insects, these appendages undergo high degree of specialization to produce different types of mouth parts and reduction of legs into only three pairs.
Centralization of nervous system into brain rather than having segmental ganglia and increased complexity of this organ in molluscs and later in vertebrates is a typical anagenetic event. Improved and complex central nervous system gave the mammals an edge over reptiles in competition and in man it conferred the ability to change the environment.
In man Anagenesis brought about increased complexity and rationalisation of brain in ape-like ancestors which were both arboreal and terrestrial and omnivorous in diet. Terrestrial habit and group selection promoted Anagenesis and complexity of brain conferred plasticity of hands, development of speech and social life.
Acquisition of homoeothermy in mammals and birds provided them with plasticity to explore areas from arctic region to deserts, which could not be conquered by their ancestors.
There is no general trend in Anagenesis. Natural selection produces patterns which determine line of evolution. Sometimes regressive development and simplification of unnecessary parts is a sign of progression and improved efficiency. For example, loss of limbs in snakes, limbless lizards and Apoda is progressive specialization in relation to their habitat. Anagenesis produces degeneration in organs of sessile and parasitic animals, but degeneration takes place only in those organs that are unnecessary for parasitic life.
A very rapid Anagenesis is termed Tachytely, while moderate speed of evolution in a lineage is called Horotely. A very slow rate of anagenesis which normally occurs in stable environmental conditions in isolated pockets is known as Bradytely. Evolutionary changes taking place per unit of time gives an idea how fast evolution has occurred in a lineage or in different comparable lineages.
Changes in a single lineage can also be defined by taxonomic categories, such as from races to subspecies and from species to genus and so on. In molecular evolution, genetic changes can be measured as rates of base substitution per unit of time. Changes in ecological conditions such as habits and habitats of the animals can also be recorded for comparative study.
Anagenetic rate = log (final value) – log (initial value)/ T2 – T1
Where T1 is the time when ancestor was present and T2 is the time when descendant occurred.
(The branching evolution)
Splitting of a lineage into two or more branches during evolution is called cladogenesis. This is one of the most common modes of speciation in changing environment or in a mosaic of environmental conditions. Species tend to break into populations and try to adapt to different environmental conditions thus setting off branching of the ancestral taxon into lower ranked ones.
Rapid splitting of lineages, often due to invasion of new environment, or due to extinction of competitors, which makes new opportunities available to the population. Natural selection is relaxed in such situations and new types can be produced very fast, leading to adaptive radiation. A split in the pseudosuchia lineage of thecodont reptiles produced birds which invaded a new environment of flying in the air, while another branch of thecodont reptiles gave rise to mammals.
Tachyschizia can also be seen in Darwin’s finches which have all branched off from a single population that was blown off South American coast to the Galapagos Islands, where ecological niches were vacant and natural selection relaxed.
This is moderate rate of splitting of phyletic lines under moderately changing environmental conditions when natural selection is not too harsh. Splitting takes place after long intervals.
A lineage persists without splitting for a long time and the evolution is slow, but a split can happen after a long gap and some branches may survive while the others become extinct. The rate of speciation can be measured as an average of several branching lineages, which would include both surviving and extinct ones.
Cladograms that show branching lineages are developed with the help of computers and compared with actual phylogenies to arrive at a realistic conclusion.